This invention relates to broadcast and high-power electromagnetic-signal amplifiers including incidental phase correction responsive to the amplitude modulation.
Television broadcast transmitters amplitude-modulate video information signals onto carriers at predetermined standard frequencies. Such transmitters and their associated antennas are normally located where the best possible broadcast coverage can be obtained. Frequently, the transmitter site is remote from the studio where the video signals are generated. Consequently, remote control of the most important transmitter parameters is required. Ideally, a remote control, such as that of transmitter output power, should not cause a degradation of the transmitted signal in other respects.
Broadcast transmitters often include an exciter which modulates video onto a carrier at the desired output frequency, but at a power level less than that desired. High-power radio-frequency amplifiers are used to amplify the output of the exciter and apply it to the antennas. The voltage and current excursions in the high power amplifiers are large, and consequently distortions of the transmitter output occur. It is customary to predistort the exciter output to compensate for the expected distortion in the high power amplifier stages. Thus, it is known to use resistor-diode networks to correct the exciter signal to compensate for differential gain, i.e., gain changes at particular video frequencies which are dependent upon the amplitude of low-frequency video signal components. It is also known to use resistor-diode networks to compensate for differential phase distortion, i.e., phase changes in high-frequency video signals resulting from low-frequency amplitude excursions. One form of distortion which is particularly severe in high-power radio frequency amplifiers is incidental phase distortion, in which the phase of the carrier passing through the high power stage is shifted in an amount depending upon the signal amplitude. This is in effect a variable delay dependent upon signal amplitude, and is believed to result from changes in the effective capacitance and/or the input impedance of the amplifiers with signal level.
It is known to use a controllable phase modulator coupled between the carrier signal generator and the output modulator of the exciter and to apply video from the video source to the control input of the phase modulator in order to predistort the exciter output to compensate for the expected incidental phase modulation in the high-power amplifiers. In this arrangement, no possibility for simultaneous adjustment of transmitter output power and incidental phase correction exists.
It is also known to modulate the video onto an intermediate-frequency (IF) carrier, which in turn is used to drive a second modulator or mixer which converts the signal to a high frequency for driving the transmitter power amplifier device. In this arrangement, both the differential gain correction and the phase modulator for incidental phase correction operate on the modulated IF carrier. The video signal required for driving the phase control terminal of the phase modulator to achieve incidental phase correction is reconstituted from the modulated IF carrier by a video detector to which a sample of the modulated IF carrier is applied. In this arrangement, the limited bandwidth of the phase modulator may undesirably attenuate the band edges of the modulated IF carrier. Also, unwanted amplitude modulation of the modulated IF carrier may occur in response to the reconstituted video control signal because of unavoidable amplitude responses of the phase modulator.
It is desirable to operate the exciter in a manner which allows remote control of the transmitter output power without degradation of the incidental phase correction. It is also desirable to operate an exciter in a manner which reduces the effect of bandwidth limitations of the phase modulator on the output signal. Similarly, it is advantageous to have a phase modulator which is smaller and which provides more control range than heretofore.